Problem: Christopher starts counting at 36, and he counts by threes. If 36 is the 1st number that Christopher counts. what is the 15th number that he counts?
Answer: What is the first number that he counts? $36$ What is the second number that he counts? $\begin{align*}&36 + 3 \\ &= 39\end{align*}$ What is the third number that he counts? $\begin{align*}&36 + 3 + 3 \\ &= 36 + (2 \times 3) \\ &= 42\end{align*}$ What is the 15th number that he counts? $\begin{align*}&36 + (14\times3) \\ &= 36 + 42 \\ &= 78\end{align*}$